(1) Field of the Invention
The present invention relates to helicopter rotor lead-lag dampers and, more particularly, to lag dampers for damping helicopter rotors in lag mode by employing a combination of controllable magnetorheological (MR) fluids and reliable viscoelastic materials.
(2) Description of Prior Art
Most modern helicopter main rotors are equipped with lead-lag dampers to alleviate aeromechanical instabilities, such as ground resonance resulting from the interaction of lightly damped regressing rotor blade lag modes with support modes. Conventional lag dampers use passive materials, such as elastomers, to dissipate energy, but their damping and stiffness levels diminish markedly as amplitude of damper motion increases. In forward flight conditions, the blade lead-lag motion in helicopters occurs at two frequencies of the lead-lag frequency and 1/rev frequency, and the large motions at 1/rev will reduce the damping at lag/rev substantially, thus, causing undesirable limit cycle oscillations. F. F. Felker, B. H. Lau, S. McLaughlin, and W. Johnson, Nonlinear behavior of an elastomeric lag damper undergoing dual-frequency motion and its effect on rotor dynamics, J. American Helicopter Society (1987) pp. 45-53. Moreover, damping augmentation is only required over certain flight regimes where there is a potential for instabilities to occur, and a passive damper providing a fixed damping may produce unfavorably large periodic loads on the rotor hub. Additionally, the mechanical properties of different dampers should be matched to minimize the impact of varying damper mechanical properties on rotor tracking conditions. “Characterization of Magnetorheological Helicopter Lag Dampers” by Kamath, Gopalakrishna, University of Maryland, Wereley, N.; Jolly, M., Journal of The American Helicopter Society (1999) July 44, 3.
Clearly, an adaptable damper, which could produce the desired amount of damping without a corresponding increase in periodic loads and could be adjusted to compensate for damping and other performance losses at extreme environmental conditions, would be of considerable value.
Magnetorheological (MR) fluid as a smart material has been proposed as the working fluid in helicopter rotor lag dampers. Hysteresis Modeling of Semi-Active Magnetorheological Helicopter Dampers, Wereley et al. Journal of Intelligent Material Systems and Structures, Vol. 10, No. 8, 624-633 (1999). Since the yield stress of the fluid demonstrates a substantial variation with the application of a magnetic field, many MR dampers for shock and vibration isolation mounts have been disclosed such that the damping level can be controlled in feedback by applying a magnetic field. See, for example, U.S. Pat. No. 5,277,281 to J. D. Carlson et al., U.S. Pat. No. 6,279,700 to H. Lisenkser et al., U.S. Pat. No. 6,311,810 to P. N. Hopkins et al., U.S. Pat. No. 6,694,856 to P. C. Chen and N. M. Wereley, and U.S. Pat. No. 6,953,108 to E. N. Ederfass and B. Banks. Much work has been done to evaluate the capabilities of MR lag dampers. Kamath et al. demonstrated the feasibility of using MR dampers for lag mode damping applications. Kamath, G. M., Wereley, N. M., and Jolly, M. R., “Analysis and testing of a model-scale magnetorheological fluid helicopter lag mode damper,” Proceedings of the 53rd Annual Forum, American Helicopter Society, Alexandria, 1997. Lag damping control using MR dampers is also under consideration. It has been shown that the ground resonance instability and damping load in forward flight can be alleviated with semi-active feedback control using feedback linearization strategies. Marathe, S., Wang, K. W., and Gandhi, F., “The Effect of Model Uncertainty on Magnetorheological Fluid Damper Based Systems Under Feedback Linearization Control,” Proceedings of the ASME International Mechanical Engineering Congress & Exposition (Adaptive Structures and Material Systems), Anaheim, Calif., November 1998, AD-Vol. 57, pp. 129-140. The controllable damping provided significant flexibility in damping augmentation strategies. However, prior efforts are based on scaled or theoretical models of MR dampers.
The combination of elastomeric materials and MR fluids in a lag damper has been considered as a rational choice. First, elastomeric materials can contribute stiffness to the lead-lag mode of blades. Second, an elastomer itself can act as a flexible sealant material to eliminate the possibility of leakage. Third, the kinematic complexity in modern bearingless or hingeless helicopter main rotors requires a flexible damper body such that damper chamber is usually made from a laminated stack of alternating elastomeric-metallic rings, and the flexible damper body provides a housing for damping fluids or MR fluids (Refs. Kamath, Panda). The feasibility of a combination of MR fluids and elastomeric materials was studied by an emulation of a magnetorheological fluid and elastic (MRFE) composite damper. W. Hu and N. M. Wereley, 2005, “Magnetorheological Fluid and Elastomeric Lag Damper for Helicopter Stability Augmentation.” International Journal of Modern Physics Part B. Vol. 19, No. 7-9, pp. 1471-1477. This experimental feasibility study validated a considerable damping control range provided by a flow mode MR valve in the MRFE damper. While damping is provided by the combination of the elastomer and MR fluid, this preliminary MRFE damper can actively augment damping over critical frequency ranges and enhance the stability of helicopter rotors. Although the stiffness in the elastomer is still available as a design parameter, the MR and elastomeric damping elements of the MRFE damper can augment each other. In addition, the passive damping in both the elastomer and MR damping elements provides a fail-safe damping in the event that control of the field-dependent MR damping is lost.
There is scarce published research on development of MRFE dampers. Description for a hybrid fluid and elastomeric damper can be found in U.S. Pat. No. 5,501,434 to D. P. McGuire. A scheme for combining an MR valve with elastomers was also disclosed in U.S. Pat. No. 5,277,281 to J. D. Carlson et al.
The present inventors propose a snubber type and a concentric bearing type lead-lag damper, both types of dampers incorporating an MR valve into a damper body. As disclosed below in further detail, the snubber type MRFE damper comprises a flexible damper body that can be made from a laminated stack of alternating elastomeric-metallic rings, a center or interior wall dividing the body into two fluid chambers, and an MR valve housed in the center or interior wall or in an external flow port. In a concentric bearing MRFE damper, elastomeric material is injected and cured in the annular gap between a pair of concentric tubes, and an MR fluid reservoir, as well as a piston-mounted MR valve, is housed inside the interior volume of the innermost tube. The fluid reservoir is fixed relative to the inner tube, and the piston is fixed relative to the outer tube. The key benefits and payoffs of the proposed MRFE technology are as follows:                Eliminates the detrimental effects of amplitude dependent damping loss at both very low amplitudes (below 0.5% strain) and high amplitudes (above 10% strain)        Adjusts damping to augment stability and performance as a function of flight condition        Adjusts damping to mitigate temperature-dependent stiffening and softening at low and high temperatures, respectively.        MRFE damper technology has no (or fewer) moving parts, offering increased reliability        Passive damping for fail-safe, reduced power, or no power operation        Retro-fit capable system, controlled/powered through existing rotor de-icing slip ring        Possible applications extend beyond rotary wing vehicles to fixed-wing and unmanned (air) vehicle applications        
Other features, advantages and characteristics of the present invention will become apparent after the following detailed description.